1. Field of the Invention
The present invention relates to a CCD image sensor. More particularly, the present invention relates to a CCD image sensor having a charge storage section between a photodiode section and a charge transfer section, the charge storage section being used to store electric charges (called hereinafter “charges”) obtained through photoelectric conversion by the photodiode section.
2. Description of Related Art
Of such CCD image sensors, Japanese Patent No. 2937192 discloses such a CCD image sensor that has a photodiode for photoelectrically converting the received light and a storage section for controlling storage of charges obtained through photoelectric conversion. In this conventional CCD image sensor, the surface of each of a silicon substrate of the photodiode and the storage section is N-type in the active state.
Furthermore, a barrier gate is provided between the photodiode and the storage section. The barrier gate receives a constant positive voltage, and is not used to control storage of signal charges generated in the photodiode. Moreover, a constant positive voltage is applied to the storage section, and the storage section including even the surface of the substrate is N-type in the active state.
FIG. 16 shows an example of a CCD image sensor according to another prior art, where a single CCD 8 is provided with multiple image areas 2-1 and 2-2 each including multiple photodiodes 2. Different image signals are inputted into the image areas 2-1 and 2-2, and signal charges are generated in the image areas 2-1 and 2-2, respectively. Control gates 4-1 and 4-2 each independently control a storage time, and the generated signal charges are stored in storage sections 5.
Japanese Patent Application Laid-open Publication No. Hei 6-216363 discloses a CCD image sensor capable of reducing a dark current. FIG. 17 is a view showing a configuration of the CCD image sensor described in this Patent document. A CCD image sensor 1 has a structure to store charges generated at photodiodes 2 in storage gates 5. In this structure, a potential barrier or the like, which blocks charge transfer from the photodiodes 2 to the CCD 8, is not generated. In addition, negative voltages are applied to the storage gates 5.
Here, descriptions will be given of the operation of the CCD image sensor 1 shown in FIG. 17, with reference to FIGS. 18A, 18B and 19. FIG. 18A is a cross-sectional view of the CCD image sensor, taken along the line X-X′ in FIG. 17. FIG. 18B shows a potential of each part in FIG. 18A. Furthermore, FIG. 19 is a view showing a potential profile of each part (the photodiode section, the storage section, and a transfer gate section) of the CCD image sensor in a depth direction. All the charges generated in the photodiode 2 are stored in the storage gate 5. As shown in FIG. 19, a negative voltage is applied to the storage gate 5 so as to invert the substrate surface to P-type. In the storage gate 5 in the PIN-ing state, a part of the semiconductor in contact with the gate oxide film is inverted to P-type, and the surface potential is a GND potential.
Moreover, there is another CCD image sensor described in Japanese Patent Application Laid-open Publication No. 2004-112797 as an application example of a structure including a charge storage section. This corrects spatial line differences of multiple photoreceptors, by providing an analog memory between the photoreceptor and a shift register as a charge transfer section.
There is a device for optically measuring a distance to a measurement target as another application example of the structure including a charge storage section, as shown in Japanese Patent Application Laid-open Publication No. 2000-249907. This is particularly used as a sensor for an auto focus (AF) function of a digital camera.